Optical detection of planarization, breakthrough and end-point in membrane-mediated electropolishing of metal layers

ABSTRACT

Methods and apparatus for monitoring in situ the progress of planarization and/or removal of a thin metal layer from a substrate during membrane-mediated electropolishing processes are provided.

FIELD OF THE INVENTION

The invention is directed to methods and apparatuses for monitoring in situ the progress of planarization and/or removal of a thin metal layer from a substrate during membrane-mediated electropolishing processes.

BACKGROUND

Interconnections on integrated circuits are fabricated by the Cu damascene process in which the interconnect circuit pattern is lithographically etched into the surface of a dielectric layer on the surface of the wafer. The pattern is then coated with thin conformal layers of a barrier metal, such as Ta, followed by Cu. Additional Cu in then electroplated over the entire surface of a wafer to fill completely the recessed circuit features with Cu. It is then necessary to planarize topographic features on the surface of the electroplated Cu and remove all excess Cu from the plateau areas between the circuit features so that the underlying barrier metal is exposed. Planarization and removal of excess Cu is customarily achieved by a chemical mechanical polishing (CMP) process. In CMP, the wafer is mechanically polished with a soft polymeric pad in the presence of an aqueous slurry containing oxidizing agents and abrasive colloid particles.

Because of the critical tolerances required for IC interconnects, it is desirable during Cu CMP to detect and/or monitor changes in the surface of the wafer during the course of polishing. In the first stage of CMP, topographic features are planarized prior to removal of Cu from any of the barrier surface. In the second stage of CMP, excess Cu is removed to the point of breakthrough, where portions of the barrier layer first become exposed, and then the Cu is cleared entirely from the surface of plateau areas separating the circuit feature(s). The process is stopped just beyond this end-point, before significant loss of Cu occurs from within the circuit features.

In CMP the surface of the wafer is continuously flooded by a polishing slurry containing suspended colloidal particles that scatter light and complicate optical measurements. Moreover, the CMP polishing pad comprises an optically opaque material. Consequently, optical measurements can be difficult to carry out without significant modifications of the polishing pad and the platen that supports the pad.

Various methods have been developed to monitor in situ the progress of the CMP process. These methods vary in sensitivity and reliability and in the cost and complexity they contribute to the equipment. For example, Thomas Tucker (p. 106 in “Chemical-Mechanical Planarization of Semiconductor Materials”, Edited by M.R. Oliver, Springer-Verlag, Berlin, 2003) states: “Optical reflectometry is the preferred method for endpointing CMP processes. However, the method is highly intrusive, requiring process compromises or extensive hardware modifications.” Such modifications are described in U.S. Pat. No. 5,893,796; U.S. Pat. No. 5,964,643; U.S. Pat. No. 6,045,439; U.S. Pat. No. 6,280,290; and U.S. Pat. No. 6,454,630.

Alternate approaches to planarization and removal of excess Cu have been disclosed by S. Mazur et al., (co-pending applications U.S. Ser. No. 10/976,897; U.S. Ser. No. 10/986,048; and U.S. Ser. No. 11/291,697, which are hereby incorporated by reference). The disclosed approaches involve a membrane-mediated electropolishing (MMEP) process. Because all metal removed from the work-piece is re-deposited onto the cathode, the MMEP process occurs with substantially no change in the composition or physical properties of the low-conductivity solvent, the membrane or the electrolyte solution.

Since MMEP consumes no reagents and generates no waste it provides advantages over CMP. However, there remains a need for methods to monitor the progress of planarization and/or to detect breakthrough and end-point in apparatus designed for the MMEP process. In particular, there is a need for an in situ method to monitor the planarization of topographic features, and also to detect breakthrough and end-point in electropolishing Cu damascene wafers by the MMEP process.

SUMMARY OF THE INVENTION

One aspect of this invention is an apparatus for use in membrane-mediated electropolishing a metal-coated workpiece comprising:

A. a cathode half-cell comprising:

-   1) a fully or partially enclosed volume, cavity or vessel; -   2) a charge-selective ion-conducting membrane having an internal     surface and an external surface, wherein the membrane forms a     surface of the enclosed volume, cavity or vessel; -   3) an electrolyte composition which at least partially fills the     enclosed volume, cavity or vessel, wherein at least a portion of the     internal surface of the membrane contacts the electrolyte     composition and at least a portion of the external surface of the     membrane contacts the work-piece during electropolishing; and -   4) an electrode in contact with the electrolyte composition; and

B. an optical detection system comprising:

-   1) a light source for irradiating the workpiece; and -   2) a means for detecting light that is reflected from the workpiece.

Another aspect of this invention is a process comprising: providing a source of light to illuminate a workpiece; illuminating the workpiece with light from the light source; detecting light reflected from the workpiece; and analyzing the reflected light to determine the topography of the workpiece.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic cross-section of MMEP apparatus with a bifurcated optic fiber incorporated in the cathode half-cell to detect changes in reflectivity of the work-piece.

FIG. 2 is a schematic cross-section of MMEP apparatus with a bifurcated optic fiber incorporated outside the cathode half-cell to detect changes in reflectivity of the work-piece.

DETAILED DESCRIPTION

This invention is directed to optical methods and apparatuses for detecting in situ changes in the topography of a work-piece during the progress of MMEP. The optical apparatuses and methods can be used to detect in situ the point of breakthrough, where a thin metal coating is first cleared from certain areas of a substrate. The apparatuses and methods can also be used to detect in situ the end-point, where a thin metal coating has been entirely removed from raised, plateau areas on the substrate surface, prior to removal by MMEP of the metal coating from recessed areas of the substrate.

In an MMEP process, the workpiece is physically separated from the electrolyte and cathode by a charge-selective ion-conducting membrane, wherein the membrane is essentially impermeable to the electrolyte but permeable to the ions produced by anodic oxidation of the work piece. MMEP is described in co-pending applications (U.S. Ser. No. 10/976,897; U.S. Ser. No. 10/986,048; and U.S. Ser. No. 11/291,697).

In MMEP, the low-conductivity solvent that covers the wafer, the membrane, and the electrolyte can be selected so as to be optically transparent. As a consequence, one can easily detect light that is reflected off the surface of the workpiece and transmitted through the membrane. In one embodiment of this invention, an optical detector is placed inside the half-cell, near the portion of the membrane in contact with the workpiece, to monitor the change in reflectivity of the surface during the MMEP process. Changes in reflectivity can be correlated with changes in topography of the workpiece surface, e.g., its roughness, as well as changes in composition of the surface, e.g., as the barrier layer beneath a Cu layer is exposed. The changes in topography and composition can be used to determine the extent of planarization of the surface.

While optical detection systems with various designs and principles of operation can be employed in the processes of this invention, optical reflectivity measurements are especially useful for this purpose. In one embodiment, the optical detection system employs bifurcated optical fibers to detect reflectivity in situ during MMEP. Such an embodiment is illustrated in FIG. 1, which shows a schematic cross-section of an MMEP apparatus in which a bifurcated pair of optical fibers 9, 10 has been incorporated within the electrolyte cavity 8 of the cathode half-cell 4. The ends of the optical fibers are positioned near the inside surface of the membrane 7 opposite the plated Cu layer 5 on the surface of the wafer 6 (the workpiece). The wafer is flooded by deionized water 1. Under the influence of the interfacial velocity v, a thin layer of deionized water is maintained at the interface between the Cu layer and the membrane by hydrodynamic lubrication forces within the contour area A_(c). The electrolyte 8 is optically transparent, and in one embodiment comprises a homogeneous aqueous solution of sulfuric acid and a Cu salt. The membrane 7 comprises an optically transparent charge-selective ion-conducting membrane. Light is supplied to the transmission fiber 9 from an external light source (not shown), and any light that is reflected back into the receiving fiber 10 is detected by an external photovoltaic transducer (not shown). When the intensity of light in fiber 9 is constant, variations in the optical reflectivity of the Cu surface 5 are manifest by corresponding changes in the intensity of light detected by the photovoltaic transducer. Since the composition and optical properties of the water, membrane and electrolyte are unaffected by the MMEP process, they do not interfere with the measurements and have little effect on the photovoltaic signal. Scratches or embossed patterns on the membrane surface may attenuate the light beams, but this effect will not change with time, and can therefore be ignored in comparison with the time-dependent effects of the electropolishing process.

Variations in reflectivity detected by the optical detection system can derive from two different types of physical effects. When the surface is rough, or contains topographic features corresponding to the circuit features, the intensity of reflected light captured by the receiving fiber 10 is attenuated by deflection, diffraction and/or scatting of the light. Therefore, as topographic features are removed in the course of the MMEP process, the time-averaged intensity of reflected light that is captured by the receiving fiber increases. This is one way in which reflectivity measurements can be used to monitor the progress of planarization and electropolishing.

The time-dependent fluctuations in the reflected light can also be monitored as the surface of the work-piece passes by the bifurcated optical fiber detector. The reflected intensity fluctuates at a frequency proportional to the interfacial velocity v due to the variations in deflection, diffraction and scattering of the light beam by the topographic features on the work-piece. During electropolishing, as the amplitude of the topographic features decreases the amplitude of fluctuations in the reflected intensity will also decrease. Therefore, monitoring the amplitude of the time-dependent fluctuations in the reflected intensity provides a way to monitor the planarization of the work-piece.

Changes in the surface composition can also be monitored. For example, the optical reflectivity of a Ta barrier layer differs from that of an equivalently smooth Cu surface. Depending upon the wavelength of light, and depending upon the thickness of Ta oxides which develop on exposed surfaces of the barrier layer, the intensity of light reflected from the exposed barrier surface may be either higher or lower than that reflected from an equivalently smooth Cu surface. This effect can be used to detect the point of breakthrough, in which the barrier layer (e.g., Ta) is first exposed on certain areas of the wafer, and also the end-point, in which Cu has been cleared from all plateau areas between the circuit elements. The optical contrast between Cu and the barrier layer can be enhanced by a suitable choice of optical band-pass filter located either after the light source or before the detector.

FIG. 1 shows a preferred embodiment in which the optical detection system is incorporated inside the cathode half-cell so that the light passes through the electrolyte solution and membrane before and after reflecting from the surface of the work-piece. This configuration is useful because the membrane and electrolyte solution can be chosen to be optically transparent, but other configurations can also be employed. In other embodiments, the components of the optical detection system can be mounted remotely from the cathode half-cell or on a portion of the cathode half-cell situated opposite areas of the work-piece, which are momentarily not within the contour area. In that case the light passes through the layer of water on the surface of the work-piece and it is not necessary that either the membrane or the electrolyte be optically transparent.

By incorporating the detector inside the half-cell, the contour area can be made arbitrarily large, for example, up to the entire area of the work piece. By locating the area of optical inspection within the contour area, the signal from the detector reflects the process at the actual time and location at which it occurs. Also, the thickness of the layer of low-conductivity solvent within the contour area is much thinner, more uniform and more stable than it is on free surfaces outside the contour area. The thickness of the water layer on free surfaces outside the contour area is non-uniform and fluctuates with time, which may produce noisy signals for reflectance measurements at those locations.

While the embodiment illustrated in FIG. 1 employs a bifurcated optical fiber for the detector, other types of optical detectors may also be used to provide a stable source of illumination and to capture and measure the intensity of reflected light. Optical detectors for use in the invention are commercially available. An example of an optical system designed to detect the intensity of light reflected from a surface by bifurcated optical fibers is the Fotonic Sensor (MTI Instruments Inc., Albany, N.Y.). FIG. 1 illustrates an embodiment of the invention using a Fotonic Sensor or an optical system similar to the Fontonic Sensor. The Fotonic Sensor is designed so that the intensity of the light collected by a return fiber 10 is governed both by the reflectivity of the surface and also by the average distance between the surface and the ends of the delivery and return fibers 9 and 10 respectively. The sensitivity to distance is caused by divergence of the light source as it emerges from the delivery fiber 9. By this principle, distance changes less than 100 nm can be detected. Thus the signal from the Fotonic Sensor changes with changes in reflectivity and/or changes in distance from the surface of the work-piece.

The light source should provide a stable source of light, both with respect to intensity and wavelength.

To avoid damaging the detector (sensor) and/or the membrane, it is preferred to position 9 and 10 so as not to touch the internal surface of the membrane.

One embodiment of the present invention provides a way to follow the progress of the MMEP process by monitoring changes in the optical reflectance of the work-piece due to changes in the surface topography and/or changes in the average composition of the surface as Cu is removed to expose the underlying barrier layer. A cathode half-cell is provided, which comprises a fully or partially enclosed volume, cavity or vessel. The half-cell also contains an electrode in contact with an electrolyte solution or gel, and is sealed on at least one surface with a charge-selective ion-conducting membrane. Preferably, the electrolyte in the half-cell is maintained at a hydrostatic pressure greater than ambient atmospheric pressure, and the membrane is sufficiently flexible to expand under the influence of this pressure to establish a convex external surface (a “bulge” or “blister”) to contact the work piece. A source of DC electrical power is connected between the work piece (which functions as the anode) and the electrode in the cell (which functions as the cathode). Electropolishing is accomplished when a portion of the external surface of the ion-conducting membrane is pressed against a portion of the work piece otherwise covered by the low-conductivity fluid, and this interfacial area is moved across the surface of the work piece. The workpiece can be held stationary and the membrane moved across its surface, or the half-cell can be held stationary and the work-piece moved, or both the work piece and the half-cell can be in motion, provided that the area of contact is not static. As used herein, “contact” (of the membrane and the workpiece) means that preferably the workpiece and the membrane are within close proximity, e.g., between 1 nm and 1 micron. When a suitable voltage is applied between the anode and cathode under such conditions, some of the metal becomes oxidized to form solvated metal ions that migrate across the membrane into the half-cell.

In the MMEP process, the external surface of the membrane used in the cathode half-cell can be topographically patterned to create a plurality of “lands”, locally projecting substantially flat areas, surrounded by locally recessed channels, which extend to the edges of the contour area A_(c). Suitable topographic patterns comprise substantially flat projecting lands less than 10 mm wider, preferably less than 2 mm wide, surrounded by recessed channels greater than 1 μm deep, preferably greater than 5 μm deep, which extend to the edges of the contour area A_(c).

A convenient way to prepare a topographically patterned membrane for use in this invention is to permanently emboss a desired pattern into the surface of a pre-existing smooth membrane. The embossing tool can consist of a screen, grid, mesh, pierced, porous, or woven sheet or expanded lattice composed of metal, glass, polymeric or natural fibers or any other material sufficiently hard to permanently deform the surface of the membrane when the two surfaces are pressed together under a suitable combination of temperature and pressure. The topographic pattern embossed into the membrane surface in this way corresponds to a cast of the embossing tool. In this, or any other operation it is desired that no holes or tears occur, which would compromise the ability of the membrane to serve as a barrier between the electrolyte solution and low-conductivity fluid.

For MMEP, suitable membranes are substantially impermeable to the electrolyte, but permeable by the solvated or complexed metal ions produced in anodic oxidation of the work piece. If these ions are positively charged, then the membrane is a cation-conducting membrane. Similarly, under conditions where anodic oxidation produces a complex metal anion, the membrane is an anion-conducting membrane.

Charge-selective ion-conducting membranes are generally solid, film-forming organic polymers that bear covalently bound ionic functional groups. The bound ions constitute fixed charges that are balanced by unbound, mobile counter-ions of the opposite charge. The latter may diffuse within the membrane or migrate under the influence of an electric field to carry electric current. Small ions in adjacent solutions, having the same sign as the mobile counter-ions, exchange readily with those in the membrane. By contrast, ions in adjacent solution with the same charge as the fixed ions in the membrane tend to be excluded from such membranes due to electrostatic repulsion. Thus, all charge-selective ion-conducting membranes are to some extent impermeable to electrolyte solutions due to exclusion of ions that share the same sign as the fixed charges.

Suitable charge-selective ion-conducting membranes include film-forming ionic polymers that are stable under the conditions of the electropolishing process. For some embodiments of this invention, suitable membranes are also optically transparent. Ionic polymers that are useful as fuel-cell membranes may also be useful as membranes in this invention.

A preferred class of membranes is cation-conducting membranes, especially those formed from polymeric ionomers functionalized with strong acid groups, i.e., acid groups with a pKa of less than 3. Sulfonic acid groups are preferred strong acid groups. Preferred polymeric ionomers are copolymers of fluorinated and/or perfluorinated olefins and monomers containing strong acid groups.

Perfluorosulfonate ionomer membranes (e.g., Nafion® membranes, E.I. du Pont de Nemours, Inc., Wilmington, Del.) contain fluorocarbon chains bearing highly acidic sulfonic acid groups. On exposure to water, the acid groups ionize, leaving fixed sulfonate anions and mobile hydrated protons. The protons may be readily exchanged with various metal cations. Perfluorosulfonate ionomer membranes are particularly well-suited for use in MMEP and in the apparatuses and processes of this invention due to their strong common-ion exclusion, high conductivity, strong acidity, chemical stability and robust mechanical properties. Perfluorosulfonate ionomer membranes are also well-suited to the present invention because they are optically transparent and colorless. Also, perfluorosulfonate ionomer membranes are easily patterned by common thermoforming techniques.

The low-conductivity solvent or solution used in MMEP contacts both the anode and the external surface of the membrane. To limit the anodic dissolution reaction, the electrical conductivity of the solvent or solution is relatively low, preferably less than 500 μS/cm. The solvent or some component of the solution should be capable of solvating or otherwise coordinating the metal ions produced at the anode to create an ionic complex that is soluble and mobile within the membrane. In some instances, it is preferable to use aqueous acetonitrile or a very dilute aqueous acid solution as the low conductivity solution rather than deionized water. For use in the apparatuses and processes disclosed herein, the low-conductivity fluid is optically transparent to the wavelengths of light used for the reflectivity measurements.

The electrolyte is chosen to provide high solubility for the metal ions or coordinated metal ions produced by oxidation of the anode, and to have sufficiently high conductivity to carry current densities up to several hundred mA/cm² without introducing significant voltage drop or heating. Conductivities of at least 100 mS/cm are preferred. The most common EP electrolytes are concentrated aqueous solutions of strong acids. For many typical anode compositions suitable for use in MMEP processes, corresponding suitable choices for cathode and electrolyte compositions can be found in Electroplating Engineering Handbook, 4^(th) Edition, pp. 100-120, by D. E. Ward, L. J. Durney, Ed., Van Nostrand Co., NY, 1984. Chloride ion has been found to be effective in maintaining the solubility of Sn⁺² and Al⁺³. A source of strong base or cyanide ion can be used in the electrolyte or in the low-conductivity solution in an anodic generation of complex metal anions such as Al(OH)₆ ⁻³ or Fe(CN)₆ ⁻³. Under these conditions, an anion-conducting membrane is used instead of a cation-conducting membrane.

The cathode can be made from any electrically conductive material that is chemically stable in the electrolyte. During electropolishing, one or more reduction reactions may occur at the cathode. In order to minimize hydrogen evolution relative to plating, metal salts, e.g., CuSO₄, can be included in the electrolyte. An electrolyte solution useful in one embodiment of the processes and apparatuses of this invention is 0.5 M CuSO₄ in 1.0 M aqueous H₂SO₄. For use in some embodiments of the processes and apparatuses of the present invention, the electrolyte is optically transparent and transmits most of the light at the wavelengths used for the reflectivity measurements. Over the wavelengths used for the measurements the optical transmittance of the electrolyte solution is typically greater than 10%, preferably greater than 50%, more preferably greater than 90%.

Membrane-mediated electropolishing processes can be used to polish and/or planarize a wide variety of metals and metal alloys. The apparatuses and processes of this invention can be used to monitor the planarization or electropolishing of a corresponding number of metals and metal alloys. Such metals and alloys include silver, nickel, cobalt, tin, aluminum, copper, lead, tantalum, titanium, iron, chromium, vanadium, manganese, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium, hafnium, tungsten, rhenium, osmium, iridium, and alloys such as brass, lead/tin alloys and steel. Preferred metals include silver, nickel, cobalt, tin, aluminum, copper, lead, and alloys such as brass, steel and lead/tin. 

1. An apparatus for use in membrane-mediated electropolishing a metal-coated workpiece comprising: A. a cathode half-cell comprising: 1) a fully or partially enclosed volume, cavity or vessel; 2) a charge-selective ion-conducting membrane having an internal surface and an external surface, wherein the membrane forms a surface of the enclosed volume, cavity or vessel; 3) an electrolyte composition which at least partially fills the enclosed volume, cavity or vessel, wherein at least a portion of the internal surface of the membrane contacts the electrolyte composition and at least a portion of the external surface of the membrane contacts the work-piece during electropolishing; and 4) an electrode in contact with the electrolyte composition; and B. an optical detection system comprising: 1) a light source for illuminating the workpiece; and 2) a means for detecting light that is reflected from the workpiece.
 2. The apparatus of claim 1, wherein the means for detecting light that is reflected from the workpiece is located within the enclosed volume of the cathode half-cell and the membrane and electrolyte are optically transparent.
 3. The apparatus of claim 1, wherein the means for detecting light that is reflected from the workpiece comprises an optical fiber and a photovoltaic transducer.
 4. The apparatus of claim 3, wherein the optical fiber is bifurcated.
 5. The apparatus of claim 1, wherein the membrane comprises a perfluorosulfonate ionomer.
 6. A process comprising: A. providing a light source for illuminating a workpiece being electropolished by MMEP; B. illuminating the workpiece with light from the light source; C. detecting light reflected from the workpiece; and D. analyzing the reflected light to determine the topography of the workpiece. 